Sparrow feeding on aphids on the Buddleia

A series following the 72 British mini-seasons of Nature’s Calendar by Kiera Chapman, Lulah Ellender, Rowan Jaines and Rebecca Warren. 

Dear Readers, I’ve done a few posts on sparrows over the past few weeks, but for many of us, the sound of the birds chirruping and generally being feisty and territorial is the very first avian sign of spring. In her chapter in Nature’s Calendar, Kiera Chapman mentions that the size of the black patch on a male sparrow’s chest is an indication of the dominance of the male: the black colouration is fed by testosterone. Furthermore, the size of a male sparrow’s testicles can increase a hundredfold, while a female sparrow’s ovaries grow over fifty times larger. But what triggers this increase?

Largish black patch?

One good suggestion would be day length – as the days grow longer, spring approaches, and with it the bounty of food (hopefully) that will feed the chicks. Something that I didn’t know (so many things I don’t know!) was that sparrows can apparently sense light through their skulls, as well as through their eyes. You don’t want to know the details of the experiment that was performed to prove this, but there we go. There are sensors in the skull of the bird that send signals to the hypothalamus, the area of the brain that controls such things as sex hormone secretion. Blind birds still had enlarged testes/ovaries as day length increased, but birds who had the skin at the top of their heads tattooed black with ink did not.

Sadly, artificial light at night seems to have a deleterious effect on many birds, including sparrows. They become active earlier in the morning and later at night, with subsequent stress, which also seems to affect their gut bacteria. Stressed birds are more likely to become diseased, which has implications for us – one possible disease that sparrows could carry is West Nile Virus. Birds are infected when bitten by mosquitoes, and then pass the disease on to humans. West Nile Virus is prevalent in the United States – though most people will have a flu-like disease for a few days, others can be more seriously affected, and may even die. Scientists have found that sparrow exposed to artificial light at night can transmit the disease to one another for a whole two days longer than sparrows kept in darker nocturnal conditions, so a sick bird could transmit the disease to many more sparrows, and potentially humans.

It’s easy to think there’s nothing we can do in the face of all the terrible things going on in the world, but one easy thing would be to turn off garden lights at night – they not only change things for birds but they confuse all kinds of night-flying insects too. Many people have chosen solar-powered garden lights, with the best of intentions, but of course these twinkle away all night. Maybe we should be thinking about digging out the candles and tea-lights instead (but let’s not set fire to ourselves or the garden eh).